Dispenser cathode structure

ABSTRACT

A dispenser MK-type cathode structure adapted for prolonged storage and handling with subsequent activation and a method of producing the same whereby a supply source material capable of yielding barium, such as alkaline-earth oxides or carbonates, i.e., barium carbonates or barium oxide, is positioned in an open-ended storage chamber of a suitable member and a porously sintered cover member seals the supply source material within the storage chamber. The cover member is composed of tungsten and includes coatings of osmium and/or rhenium on its active or emission surface. Such cover member is further provided with a tightly adhering protective coating that is vapor and humidity impermeable and is composed of a hydrophobic material, such as paraffin or methyl mathacrylate resin or other heat-vaporizable hydrophobic material.

United States Patent Hofmann et al.

3,676,731 July 11, 1972 DISPENSER CATHODE STRUCTURE Horst Hohnann; l-lelmut Kata, both of Munich, Germany Siemens Aktiengesellschalt, Berlin and Munich, Germany March 3, 1971 inventors:

Assignee:

Filed:

Appl. No.:

Int. Cl. .H0lj 1/14, H0 1 j l9/O6 Field of Search ..3 13/346 R, 346 DC References Cited UNITED STATES PATENTS 5/1968 Zalm ..3l3/346 R Primary Examiner-David Schonberg Assistant Examiner-Paul A. Sacher Attorney-Hill, Sherman, Meroni, Gross & Simpson ABSTRACT A dispenser MK-type cathode structure adapted for prolonged storage and handling with subsequent activation and a methodof producing the same whereby a supply source material capable of yielding barium, such as alkaline-earth oxides or carbonates, i.e., barium carbonates or barium oxide, is positioned in an open-ended storage chamber of a suitable member and a porously sintered cover member seals the supply source material within the storage chamber. The cover member is composed of tungsten and includes coatings of osmium and/or rhenium on its active or emission surface. Such cover member is further provided with a tightly adhering protective coating that is vapor and humidity impermeable and is composed of a hydrophobic material, such as paraffin or methyl mathacrylate resin or other heat-vaporizable hydrophobic material.

4 Claims, 1 Drawing Figure BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates todispenser cathodes and more particularly to MK-type cathode structures adapted for prolonged storage and handling in ambient atmosphere and methods of producing the same.

2. Prior Art Metal capillary cathodes (i.e., MK cathodes) are known. Generally, such cathodes comprise a confining metal cup-like member having a supply source material (for an emissionpromoting substance), such as a material yielding barium at a relatively high temperature, and a porous disk member welded to the rim of the cup-like member so'that the upper surface of such disk defines an emission surface. A heating means heats both the cup-like member and disk to a suitable high temperature causing the supply source material to release the emission promoting substance for evaporation and migration by capillary action through the porous disk. The emission-promoting substance forms a film on the emission surface of the disk and acts to reduce the work function of the material forming the disk.

In certain more recent development relating to this type of dispenser cathode, suggestions have been made to anneal a thin layer of, for example, osmium onto the emission surface of the disk so as to lower the operating temperature thereof and to decrease the work function of the resultant emission surface. However such modified MK cathodes do not function consistently and tend to exhibit undesirable deviations in the resultant lowered work function. It appears that exposure of the osmium layer to ambient carbon compounds before or during the operation of such MK cathodes almost always causes deviation in their work function.

One solution to the above problem has been to anneal a rhenium coating on top of the osmium layer. One of the advantages resulting from the use of a rhenium-osmium-tungsten disk is that an alkaline-earth carbonate supply source material can be utilized since rhenium does not lend itself to carbide formation. This allows the storage and mounting operations of MK cathodes utilizing such disks to take place in an ambient atmosphere.

Nevertheless, after considerable observation of these rhenium-osmium-tungsten disks, it now appears that the rhenium coating is sensitive to partial pressures of water vapor when the storage periods are extended to weeks and/or months. It has been noted that the rhenium coating becomes oxidized during varying periods of time, primarily depending upon the relative humidity and the length of the storage period. Of course, this has an adverse effect on the emission of the cathode, for example, during initial operation thereof, increased work-functions are observed during the first 300 to 500 hours of work.

Accordingly, it is an object of the invention to provide MK cathode structures overcoming these and other disadvantages associated with prior art structures.

SUMMARY OF THE INVENTION In its more general aspects the invention provides a MK cathode having an emission disk impervious to a high relative humidity over extended storage periods. The emission disks of the invention include tungsten disks annealed with osmium and/or rhenium. In accordance with the principles of the invention, the emission disks are provided with a tightly adherent protective coating impervious to vapor and humidity and composed of a volatilizable hydrophobic material, which is soluble in an organic solvent. Examples of such hydrophobic materials include paraffin, waxes, methyl mathacrylate resins as well as other synthetic resin material having the requisite properties.

BRIEF DESCRIPTION OF THE DRAWING The single FIGURE is a diagrammatic sectional view, partially broken away, of an embodiment of a MK cathode structure constructed in accordance with the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS I sert members of the invention are so constructed as to be readily inserted into the operational environment for immediate activation and use.

The insert members are somewhat cup-shaped so that an amount of a supply source material, which upon heating yields an emission promoting substance, can be positioned therein and suitably sealed. During operation the emission-promoting substance communicates with the active surface by traveling from the storage chamber of the insert member through the sealing member, i.e., a porous tungsten disk, which may have annealed layers of osmium and/or rhenium thereon. A layer of a protective hydrophobic material is provided on such sealing member and during operation this layer is volatilized and removed whereby steady operative conditions are readily attained.

Preferably, the supply source material is selected from the group consisting essentially of alkaline-earth metal carbonates, alkaline-earth metal oxides, barium-strontium carbonate, barium oxide and mixtures thereof. The preferred vapor protective material is a hydrophobic material which is volatilized under normal cathode operating conditions, i.e., with the application of relatively large amounts of heat, and is soluble in an organic solvent to facilitate its application to the sealing member. Such hydrophobic materials include a wide range of organic materials such as paraffins, waxes, methyl mathacrylate resins, synthetic resins and other material. Preferably, the hydrophobic materials are selected from the group consisting essentially of paraffin and methyl mathacrylate resin.

In its more broad terms, the invention provides a means of protecting an emission surface of a dispenser cathode structure from ambient atmosphere and particularly water vapor, which does not adversely eflect or react with the emission surface and which is completely removed under operating conditions of such cathodes. In distinction to prior art practices of utilizing either bare tungsten disks or of utilizing osmium and/or rheniumcovered tungsten disks, the invention utilizes such disks improved by utilizing a protective organic coating thereon. Since rhenium does not form any carbide-type compound with carbon and since diffusion of organic compounds into the tungsten base takes some time, a wide variety of organic materials are utilizable in accordance with the principles of the invention to provide the desired protection on the covering disks.

In one embodiment of the invention, desired protection is achieved by coating a suitable sealing member, i.e., a tungsten disk, with a solution of paraffin in chloroform or other suitable solvent. The protection afiorded to a porous disk by means of this embodiment is highly effective and even allows the use of barium oxide (which is quite sensitive to humidity) as a supply source material in MK-cathodes, without any adverse effects. Further, such MK cathodes which are thus rendered extremely well suited for prolonged storage, have a further improved characteristic in that no essential gas production takes place during the start of the heating process. Of course, the advantages of cathodes utilizing carbonates supply source materials and osmiumtated or rheniumtated tungsten disks are not diminished or altered and in fact make such cathodes even more advantageous.

In order to further illustrate and not limit the invention, the following examples of certain embodiments of the invention are set forth.

EXAMPLE I An amount of a purified paraflin having a solidification point of about 50 to 70 C. was dissolved in chloroform so as to obtain a saturated colorless solution at room temperature. About 0.015 milliliters of this solution was applied, as by dipping, to a surface of a circular rheniumated tungsten ,disk

having a diameter of about 3 millimeters. The disk was then dried until the solvent evaporated. A paraffin coating of about 0.25 milligrams remained tightly adhering to the surface and completely filling the pores in the disk. Such protectively coated disk was impervious to water vapor over extended periods of time and was readily removed at elevated temperatures.

Of course, in its more general aspects this embodiment comprises providing a saturated solution at room temperature of a paraffin having a solidification point of about 50 to 70 C. in chloroform and dipping a suitable cover member, i.e., an osmiumtated and/or rheniumtated tungsten disk, into such a solution and evaporating the chloroform so that a thin layer of paraffin remains tightly adherent to the cover member.

EXAMPLE I] 4.0 milliliters of Pexisol P-550 (a trademark for a methyl mathacrylate resin) was dissolved in a solvent mixture of about 1.0 milliliters of butyl acetate and 0.1 gram of 2-ethyl phthalic acid ester. About 0.1 milliliters of this solution was applied, as by plunging or submerging, to a surface of a circular rheniumated tungsten disk having a diameter of about 3 millimeters. After evaporation of the solvent mixture, a thin film of plexiglass (a tradename for methyl mathacrylate resin) remained tightly adherent to the coated surface. This protective coating wasimpervious to water vapor over extended periods of time and wasreadily removed at elevated temperatures without any adverse effect on the disk.

Of course, in its more general aspects, this embodiment comprises providing a coating solution containing a methyl mathacrylate resin in a solvent mixture of butyl acetate and 2- ethyl phthalic acid ester, submerging the disk member into the resultant solution and evaporating the solvent mixture so that a thin film of the resin material remains tightly adherent to the coated surface.

Referring now to the drawing, wherein an embodiment of the cathode structure of the invention is illustrated. Those parts thereof which do not directly contribute to a better understanding of the invention are either left out or merely not provided with reference numerals. The cathode structure illustrates, has a cupshaped member 1 with a storage chamber la that is open at its upper edge. An amount of a stock supply material, such as an alkalineearth metal carbonate or alkaline-earth metal oxide (i.e., barium'strontium carbonate or barium oxide), either in powder or tablet-like form is positioned within the storage chamber 10. A porously sintered emission-material carrier disk 3, composed of, for example, tungsten is provided with a relatively thin layer 4 of osmium and/or rhenium on its active or upper surface and then assem bled so as to seal the storage chamber la. Sealing means 2a are provided for tightly holding the carrier disk 3 in proper relation on the cup-shape member 1. A relatively thin protective coating 5 is provided on top of the layer 4 so as to completely seal it from ambient atmosphere and prevent any moisture or vapor from penetrating through the disk. The coating 5 is composed of a hydrophobic material that is soluble in an organic solvent and readily vaporizes upon the application of heat, such as that produced during normal operation of a dispenser cathode. A suitable hydrophobic material having the requisite characteristic is paraifin dissolved in chloroform, since it provides a protective coating that is impermeable to humidity and readily vaporizes under normal operatin conditions.

Thus %he invention provides a novel indirectly heated dispenser cathode structure and method of producing the same. The structure comprises a member having an open ended storage chamber for suitable supply source materials and a porous cover member of a suitable metal sealing the storage chamber and provided with a protective vapor-impermeable coating of a hydrophobic material that vaporizes under nonnal cathode operating conditions.

Various mOdifications and changes can be effected without departing from the spirit and scope and novel concepts of the invention.

We claim:

1. An MK dispenser cathode structure adapted for prolonged storage and insertion into an operational environment comprising a body member having an open ended storage chamber therein, a supplyv source material selected from the group consisting essentially of barium carbonates, barium oxides, alkaline-earth metal oxides capable of yielding barium, alkaline-earth metal carbonates capable of yielding barium and mixtures thereof positioned within said storage chamber, a porous cover member positioned on said body member so as to seal said storage chamber with one surface and have an opposite surface define an emission surface for the cathode, and a protective coating on said emission surface composed of a vapor impermeable hydrophobic material soluble in an organic solvent and volitizable under normal cathode operating conditions.

2. An MK dispenser cathode as defined in claim 1 wherein the protective coating is composed of paraffin.

3. An MK dispenser cathode as defined in claim 1 wherein the protective coating is composed of methyl mathacrylate resin.

4. An MK dispenser cathode structure adapted for prolonged storage and insertion into an operational environment comprising a body member having an open ended storage chamber therein, a supply source material capable of yielding barium positioned within said storage chamber, a porous cover member composed essentially of tungsten and including a coating of a material selected from the group consisting essentially of osmium, rhenium and mixtures thereof, along at least one surface thereof, said cover member being positioned on said body member so as to seal said storage chamber with a surface and have an opposite surface define an emission-surface for said cathode, and a protective coating on said emission surface composed of a humidity-impermeable hydrophobic material soluble in an organic solvent and volatilizable under normal cathode operating conditions selected from the group consisting essentially of parafifins, methyl mathacrylate resins and mixtures thereof. 

2. An MK dispenser cathode as defined in claim 1 wherein the protective coating is composed of paraffin.
 3. An MK dispenser cathode as defined in claim 1 wherein the protective coating is composed of methyl mathacrylate resin.
 4. An MK dispenser cathode structure adapted for prolonged storage and insertion into an operational environment comprising a body member having an open ended storage chamber therein, a supply source material capable of yielding barium positioned within said storage chamber, a porous cover member composed essentially of tungsten and including a coating of a material selected from the group consisting essentially of osmium, rhenium and mixtures thereof, along at least one surface thereof, said cover member being positioned on said body member so as to seal said storage chamber with a surface and have an opposite surface define an emission-surface for said cathode, and a protective coating on said emission surface composed of a humidity-impermeable hydrophobic material soluble in an organic solvent and volatilizable under normal cathode operating conditions selected from the group consisting essentially of paraffins, methyl mathacrylate resins and mixtures thereof. 